Adobe PHOTOSHOP 6.0 Manual page 375

For more information on choosing file formats
when opening or saving images, see "Opening and
importing images" on page 101 and "Saving
images" on page 357. For information on
choosing a Web optimization format, see
"Choosing a file format for optimization" on
page 317.
Note: If a supported file format does not appear in
the appropriate dialog box or submenu, you may
need to install the format's plug-in module.
For more information about specific
file formats and plug-in modules,
see "About file formats" in online Help.
About file compression
Many image file formats use compression
techniques to reduce the storage space required by
bitmap image data. Compression techniques are
distinguished by whether they remove detail and
color from the image. Lossless techniques
compress image data without removing detail;
lossy techniques compress images by removing
detail. The following are commonly used
compression techniques:
• Run Length Encoding (RLE) is a lossless
compression technique supported by Photoshop
and some common Windows file formats.
• Lemple-Zif-Welch (LZW) is a lossless
compression technique supported by TIFF, PDF,
GIF, and PostScript language file formats.
This technique is most useful in compressing
images that contain large areas of single color,
such as screenshots or simple paint images.
• Joint Photographic Experts Group (JPEG) is a
lossy compression technique supported by JPEG,
TIFF, PDF, and PostScript language file formats.
JPEG compression provides the best results with
continuous-tone images, such as photographs.
When you choose JPEG compression, you specify
the image quality by choosing an option from the
Quality menu, dragging the Quality pop-up slider,
or entering a value between 1 and 12 in the Quality
text box. For the best printed results, choose
maximum-quality compression. Files with JPEG
encoding can be printed only on Level 2 (or later)
PostScript printers and may not separate into
individual plates.
• CCITT encoding is a family of lossless
compression techniques for black-and-white
images that is supported by the PDF and
PostScript language file formats. (CCITT is an
abbreviation for the French spelling of Interna-
tional Telegraph and Telekeyed Consultive
Committee.)
ZIP encoding is a lossless compression technique
•
supported by the PDF and TIFF file formats.
Like LZW, ZIP compression is most effective for
images that contain large areas of single color.
(ImageReady) PackBits is a lossless compression
•
technique that uses a run-length compression
scheme. PackBits is supported by the TIFF file
format in ImageReady only.
ADOBE PHOTOSHOP 6.0 365
User Guide